I'm no expert, but I personally would go with ceramic or porcelain. I like to bleach, scrub and clean my showers with reckless abandon. I'd be too paranoid that any of the cleaners commonly used in a bathroom would discolor granite.

Actually, you should use porcelain, and keep them kinda small for the shower floor. I recommend no bigger than 3.5". Although porcelain is ceramic, when I think of ceramic I think of regular soft bisque wall tiles, which should not be used on a shower floor or any floor.